Insulating paper of good thermal conductivity

ABSTRACT

THE THERMAL CONDUCTIVITY OF ELECTRICAL INSULATION IS IMPROVED BY INCORPORATING THEREIN NON-ACTIVATED ALUMINUM HYDROXIDE POWDER HAVING A PURITY OF AT LEAST 99 PERCENT AND A MAXIMUM NA2O AND FE2O3 CONTENT OF 0.7 AND 0.05 PERCENT, RESPECTIVELY, AND/OR BY COATING OR IMPREGNATING SAME WITH AN ADMIXTURE OF RESIN, THERMALLY CONDUCTIVE MATERIAL AND SURFACE ACTIVE SUBSTANCE.

March 21, 1972 TOSHITAKA suD ETAL 3,650,954

INSULATING PAPER OF GOOD THERMAL CONDUCTIVITY Filed June 12, 1968 6 Sheets-Sheet 1 CT 50 H I b 40 O U 5'; 3O 8 4 20"- 2 Of LIJLI-I IO U I g; I (1. O O I I I I O IO 20 3O 40 SO CONTENT OF INORGANIC SUBSTANCE (WEIGHT 0) FIG .I

/\9 Z O 8O 70 X f, T5 60 I: r m 1 50 I U O I 20 3O 40 "SO CONTENT OF INORGANIC SUBSTANCE (WEIGHT /0) FIGS E 8 E is m B H1 (1. g I I I I C, U I

O IO 20 3O 4O 5O CONTENT OF INORGANIC SUBSTANCE (WEIGHT INvENTORS TOSHITAKA YASUOA F163 SHIRO KATAYAMA YUTAKA SUZUKI ATTORNEYS March 21, 1972 TOSHITAKA YASUDA ET AL INSULATING PAPER OF GOOD THERMAL CONDUCTIVITY 6 Sheets-Sheet 2 Filed June 12.

5 EATED DAYS DATE) HEATED DAYS (DATE) mmm O m2 H O O E M QB S 5 A A M D D E T 2A 4E H O O 8O HEATING TIME (HOURS) INVENTORS TOSHITAKA YASUDA SHIRO KATAYAMA YUTAKA SUZUKI BY MM, ,z/ mc' ATTORNEYS March 21, 1972 TOSHITAKA YASUDA L 3,650,954

INSULATING PAPER OF GOOD THERMAL CONDUCTIVITY Filed June 12, 1968 6 Sheets-Sheet lO- 3 g 9 7 g 27 LU 0: Lu 5 q 24 25 23 a 4 LL 0 3 2 2 I I I I o 20 40 8O HEATING TIME (HOURS) FIG.6

2 I2 IO- 2 a 1 2 28 Q3 4 i] 59 2 a O I I I I I I I I Z 0 I00 200300 400 500 600 HEATING TIME (HOURS) LL] 2 E5 $3 (I LL! 5 l 9 5 I l l l l 0 I00 2003004005006OO 700 HEATING TIME (HOURS) INVENTORS T0sHITAI A YASUDA FIG-8 sHIRo KATAYAMA YUTAKA SUZUKI BY ATTORNEYS March 21, 1972 os n- YASUDA ETAL 3,650,954

INSULATING PAPER OF GOOD THERMAL CONDUCTIVITY 6 Sheets-Sheet 5 Filed June 12, 1968 HEATING TIME (HOURS) 6 M 21 O l w w I $6-3 muzfimmmm M239 INVENTORS TOSHITAKA YASUDA SHIRO KATAYAMA YUTAKA SUZUKI ATTORNLYS Gov 8 Lo mam MEDEQEEE mo Wm March 21, 1972 TOSHITAKA YASUDA ETA!- INSULATING PAPER OF GOOD THERMAL CONDUCTIVITY Filed June 12, 1968 6 Sheets-Sheet 6 FIG.I4

INSULATING MATERIAL COMPRISING OP NON-ACTIVATED ALUMINUM HYDROXIDE POWDER.

FIG.I5

FIG.

FIG.

INSULATING MATERIAL COMPRISING OF RESIN OR VARNISH NON-ACTIVATED ALUMINUM HYDROXIDE POWDER, SURFACE ACTIVE TI-IERMALLY CONDUCTIVE MATERIAL AND ELECTRICALLY RESISTANT INORGANIC POWIIR.

INSULATING MATERIAL COMPRISING OF RESIN OR VARNISH, NON -ACTIVATED ALUMINUM HYDROXIDE POWDER AND SURFACE ACTIVE THERMALLY CONDUCTIVE MATERIAL.

INVENTORS TOSHITAKA YASUDA SHIRO KATAYAMA Y"TAKA SUZUKI ATTORNEYS US. Cl. 252-632 3 Claims ABSTRACT OF THE DISCLOSURE The thermal conductivity of electrical insulation is improved by incorporating therein non-activated aluminum hydroxide powder having a purity of at least 99 percent and a maximum Na O and Fe O content of 0.7 and 0.05 percent, respectively, and/ or by coating or impregnating same with an admixture of resin, thermally conductive material and surface active substance.

CROSS-REFERENCE TO RELATED APPLICATION This is a continuation-in-part of application Ser. No. 371,903, filed June 2, 1964, now abandoned.

BACKGROUND OF THE INVENTION Paper (including pressboard, varnished paper and nonwoven fabric) employed for electrical insulation is subject to many factors. Although considerable etiort has been directed to improve numerous properties, including the heat resistance of such paper, little has been done with respect to thermal conductivity, the property with respect to which the subject invention is directed. Since coils of all electrical instruments are wrapped by paper of inferior thermal conductivity, heat generated in a conductor (due to the Joule heat of the electric current) accumulates and the temperature rises. As a result of heat build-up (inferior thermal conductivity) and poor heat resistance, the current density of the coils is severely restricted.

Insulating paper, such as coil insulating paper and power cable insulating paper, has been prepared almost exclusively from vegetable, chemically treated vegetable or chemical fiber, varnish and resin. Attempts to improve the properties of electrical insulating paper have resulted in such heat resisting and heat insulating papers as cellulose inductor paper, chemically treated paper and paper prepared from synthetic fibers. If some inorganic material were added to or incorporated in such paper, its purpose was to improve mechanical and electrical characteristics rather than thermal conductivity. There have been numerous modifications of cellulose and other fibrous products.

French Pat. 844,945 discloses a process for manufacturing a cellulose composition containing resin (from wood pulp, wood, straw or other similar vegetable material), alkali metal hydroxide, another metal hydroxide, e.g. aluminum hydroxide, and a dispersing agent (the ingredients being added to bisulfate pulp during the digesnited States Patent tion of same) to obtain a product having an increased acellluose content for use in rayon. In order to improve the softness, flexibility, durability and strength of paper, United States Pat. 2,249,118 discloses fixing (by means of a water-insoluble filler, such as calcium, magnesium and aluminum oxides and salts) a softening agent, such as glycerine or other stable water-soluble liquid having a higher boiling point than water, in paper so that moisture will not dissolve and heat will not evaporate the softening agent and thus leave the paper in a stilf and harsh condition.

Condensers are prepared with only one sheet of paper insulation between their electrodes (USP 2,281,602) when the paper is coated with a porous coating prepared from an admixture of binder, such as polystyrene and ureaalkyd resin, and filler, such as titanium oxide, powder, aluminum oxide, silica and other insulating pigments. The thus-coated paper is an improved dielectric material which withstands much higher voltage and permits the manufacture of higher-capacity condensers.

British Pat. 631,483 is directed to the incorporation in paper stock (in the beater) of a metal hydroxide, such as hydroxides of aluminum, iron, chromium, zinc, manganese, tin, titanium and magnesium, which may be precipitated in gel-like form without the addition of some other agent to increase the wet strength of the resulting paper.

Polymerizable polyester resin, an alkyl hydrocarbon polymer and a high molecular halogenated hydrocarbon are employed (USP 2,921,867 with or without fillers, such as calcium carbonate, calcium sulfate and titanium dioxide, to produce inexpensive laminates with glass, paper, asbestos, nylon or other synthetic fiber, said laminates having flexibility, high dielectric strength and a desirable dielectric constant.

The printing properties, particularly printing on a multicolor press, are improved according to United States Pat. 2,975,147 by coating paper with a composition of binder (a polyester and an aqueous solution of a protective colloid) and kaolinitic clay.

A low bonded base sheet is impregnated (USP 3,026,241) with a composition containing a saturant (a copolymer formed from at least one polymerizable a,,3- unsaturated carboxylic acid in which the unsaturation is a double bond, or ethylenic linkage, and at least one alkyl acrylate) and various agents and fillers [c.g. calcium, zinc, barium and magnesium oxides (to improve solvent resistance, heat and light stability, dry tensile strength and the rate of wet strength development on heat aging); clay, calcium carbonate, blanc fixe and talc (as a loading or extending agent); and titanium dioxide (to increase opacity and improve whiteness)] to produce a saturated sheet having good tensile and stretch properties, as well as high wet strength, high folding endurance, high flexibility, high internal tear, high edge tear, delamination resistance and resistance to physical degradation and discoloration due to heat and light aging.

United States Pat. 3,090,705 improves the characteristic power factor/temperature curve in condensers by lowering the power factor with a view to increasing critical condenser performance by incorporating in condenser dielectric paper finely divided active substance(s), such as silicon dioxide, titanium dioxide, silicic acid and titanic acid, to bind and extract impurities from mineral oil or diphenyl chloride, with which the paper is impregnated.

According to United States Pat. 3,259,516 compositions which insulate thermally against heat and flame are prepared in situ from urethane prepolymer in admixture with a refractory filler and a cross-linking agent for the prepolymer.

SUMMARY To improve the thermal conductivity of a fiber insulator (including paper, pressboard, varnished paper, varnished non-woven fabric and varnished cloth) for electrical intion, a composition is coated on or incorporated in the fiber insulator. The composition comprises an intimate admixture of a solution of at least one resin and/or varnish, from 20 to 150 percent by weight, based on the weight of solids in the solution, of inorganic powder having good electric resistance and an average particle diameter of at most 0.1 micron, and at most percent by weight, based on the weight of the inorganic powder, of surface active thermally conductive material which reduces the thermal resistance between the solute (resin and/or varnish) and said inorganic powder.

Another embodiment comprises a uniformly filled electrical fiber insulator of improved thermal conductivity, the essential filler of which is non-activated aluminum hydroxide powder of at least 99 percent (by weight) purity and having a maximum Na O content of 0.7 percent by weight, a maximum Fe O content of 0.05 percent by weight and a maximum average particle diameter of several microns, e.g. from 0.1-3 microns. The fibers of the insulator are extensively impregnated or coated with the filler. The resulting insulator, coated with or impregnated by the first-noted composition, has further improved thermal conductivity.

Alternatively, the inorganic powder can be omitted from said first-noted composition when the filled paper is employed as the base material. When said inorganic powder is omitted, the amount of surface active agent is ordinarily less than 7.5 percent by weight, based on the weight of solids in the solution.

Improved thermal conductivity is imparted to a fiber insulator for electrical insulation without deteriorating its electrical characteristics by preparing the noted filled, coated and impregnated products.

An object of the present invention is thus to improve the thermal conductivity of a fiber insulator employed for electrical insulation without impairing the electrical characteristics thereof. A concomitant object is to prepare electrical insulations which permits the dissemination of heat generated in a conductor, thus avoiding the accumulation of heat and the losses attendant therewith. A further object is to prepare a coating or impregnating composition which improves the thermal conductivity of substrates to which it is applied and to apply such composition to appropriate substrates.

An additional object is to impart thermal conductivity to an electrical insulator by incorporating therein, as the sole essential ingredient, a non-activated aluminum hydroxide powder of a specified quality. The temperature rise in coils (insulated with insulation according to this invention) of electric instruments is lower than that of conventionally insulated coils because of the improved thermal radiation. Consequently, the electric density and output of the coils can be greater without overheating the interior of the coils.

Still further objects are apparent from the description and claims which follow.

DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates one of the presently preferred embodiments of the invention wherein an increase in thermal conductivity is denoted for insulating paper impregnated with non-activated and inactive inorganic material which is not corrosive, but is highly insulating (electrically) and has a better thermal conductivity than organic insulators. In this figure, the abscissa indicates the content of non-activated and inactive material in weight percent,

4 while the ordinate indicates the corresponding percent increase in thermal conductivity. The curves 1, 2, 3 and 4 represent the addition of aluminum hydroxide, aluminum oxide, calcium carbonate and talc, respectively.

FIG. 2 shows the AC break-down voltage of the foregoing wherein each improved insulating paper is impregnated with insulating oil. The abscissa shows the content of non-activated and inactive inorganic substance in weight percent, while the ordinate shows the AC break-down voltage. The curves 5, 6, 7 and 8 represent the insulating papers impregnated with aluminum hydroxide, aluminum oxide, calcium carbonate and talc, respectively.

FIG. 3 shows the impulse break-down voltage when insulating oil is impregnated into the said insulating paper of the foregoing embodiment. The abscissa denotes the content of non-activated and inactive inorganic substance in Weight percentage, while the ordinate represents the impulse break-down voltage. The curves 9, 10, 11 and 12 reflect the results with aluminum hydroxide, aluminum oxide, calcium carbonate and talc, respectively.

FIG. 4 illustrates the change in characteristics of insulating oil when the said insulating paper is heated in the insulating oil. In the diagrams (a) (b) and (c), the abscissa indicates the heating days and the ordinate shows the acid value, dielectric tangent and resistivity, respectively. In the same diagrams, 13, 14, 15 and 16 show the results for aluminum hydroxide, aluminum oxide, calcium carbonate and talc, respectively, while 17 is an ordinary insulating paper. Each of the impregnants or fillers, aluminum hydroxide, aluminum oxide, calcium carbonate and tale, is inactive and non-activated in all references made with respect thereto in the discussion of the drawmgs.

FIG. 5 shows the change in water extract conductivity of an ordinary kraft paper or insulating paper, the thermal conductivity of which is improved as mentioned in the foregoing embodiments, when it is heated in air. The abscissa denotes the time heated, while the ordinate, the water extract conductivity. The curves 18, 19, 20 and 21 show aluminum hydroxide, aluminum oxide, calcium carbonate and talc, respectively, while 22 is an ordinary insulating paper.

FIG. 6 shows the change of pH of water extract of the same insulating paper, when said paper is heated in air. The abscissa denotes the heating time, while the ordinate shows the pH of the water extract. The curves 23, 24, 2S and 26 represent insulating papers impregnated with aluminum hydroxide, aluminum oxide, calcium carbonate and talc, respectively, while 27 is an ordinary insulatmg paper.

FIG. 7 shows the curve 28 which denotes the change of AC insulation break-down voltage when varnished nonwoven fabric of the same good thermal conductivity as that of Example 2 is heated in air. The abscissa denotes the heating time, while the ordinate, the AC insulation break-down voltage.

FIG. 8 shows the curve 29 which denotes the change in volume resistivity when the varnished non-woven fabric of Example 2 is heated in air. The abscissa shows the heating time, while the ordinate, the volume resistivity.

FIG. 9 shows curve 30 which denotes change in AC insulation break-down voltage when varnished non-woven fabric, which has the same good thermal conductivity as the embodiment of Example 2, is placed into air having a relative humidity of The abscissa denotes moisture absorption time, While the ordinate, AC insulation break-down voltage.

FIG. 10 shows the change in volume resistivity when varnished non-woven fabric of the same good thermal conductivity as the embodiment of Example 2 is placed into air having a relative humidity of 100%. The abscissa denotes moisture absorption time, while the ordinate, volume resistivity.

FIG. 11 shows AC insulation breakdown voltage when varnished paper of the same good thermal conductivity as the embodiment of Example 4 is heated in air. The abscissa denotes heating time, while the ordinate, AC insu lation break-down voltage. Curve 32 represents the embodiment of Example 3, while curve 33 represents that of Example 4.

FIG. 12 shows the change in volume resistivity when varnished paper of the same good thermal conductivity as the embodiment of Examples 3 and 4 is heated in air. The abscissa denotes heating time, while the ordinate, volume resistivity. Curve 34 is for the embodiment of Example 3, While curve 35 is for that of Example 4.

FIG. 13 shows the comparison of average temperature rise of coils due to electric current when coil layer insulating paper made according to the present invention is used in the coil and when conventional coil layer insulating paper is used in the coil. The abscissa denotes the time, while the ordinate, the average temperature rise of the coils. Curve 36 stands for conventional insulating paper, while the curve 37 represents coil insulating paper made according to the invention.

FIGS. 14 to 17 are directed to embodiments I to IV, respectively, defined in the resume. Each represents an insulating material having in intimate contact therewith and dispersed throughout one or more of the following:

(a) resin or varnish,

(b) non-activated aluminum hydroxide powder,

(c) surface active thermally conductive material, and (d) electrically resistant inorganic powder;

Embodiment I (FIG. 14)a, c and d; Embodiment II (FIG. 15)-b; Embodiment III (FIG. 16)a, b, c and d; and Embodiment IV (FIG. 17)-a, b and c. It is preferred to have the designated substances uniformly dispersed throughout the insulating material.

DETAILS Fibrous insulators for electrical applications and having improved thermal conductivity and prepared from such substances as insulating paper, pressboard, cloth and nonwoven fabric.

Although kraft paper is in common usage for electrical insulation, numerous other fibrous products are likewise useful for this purpose. Synthetic fibers employed for this purpose, e.g. polyethylene terephthalate (dimethyl terephthalate/ethylene glycol condensate, such as Dacron) and polyamides (e-caprolactam polymer, such as nylon 6, or adipic acid/hexamethylene diamine polymer, such as nylon 66), are well known. Electrical insulators are prepared from sulfite pulp as well as from kraft pulp. Alternative vegetable fibers are such bast fibers as manila hemp, linen, kozo (Broussonetia paperifera), ramie and mitsumata (Edgeworthia papyrifera). Chemically treated vegetable fibers, e.g. cyanoethylated fiber, hydroxyethylcellulose fiber, acetate andamine-treated paper, may also be employed. The preceding are merely exemplary of fibers that may be employed for the preparation of electrical insulation, the aluminum hydroxide (inactive and non-activated) has a purity of at least 99 percent by weight, and Na O content of at most 0.7 percent by weight, and Fe O content of at most 0.05 percent by weight and an average patricle diameter of at most about several microns. Such aluminum hydroxide is incorporated in the electrical insulation substrate during the manufacture of said substarate. Thus, in the production of kraft paper for electrical insulation, e.g., the finely-divided aluminum hydroxide powder is uniformly admixed with the pulp slurry in the beater (with the rate of yield during the paper manufacturing process taken into consideration); otherwise the insulating paper is prepared conventionally.

The amount of aluminum hydroxide in the finished insulator, based on incorporation during the manufacturing process, is at most 50 percent by Weight and, preferably, at least 10 percent by weight, based on the weight of the final filled insulator. Although amounts less than 10 percent enhance the thermal conductivity of the resulting product, FIG. 1 illustrates the desirability of the preferred range and the further advantage of employing at least about 20 percent by weight of said aluminum hydroxide.

Although the average particle diameter for the nonactivated aluminum hydroxide powder is not greater than about 3 microns, it can be less than 0.05 micron. A particle diameter analysis of one suitable aluminum hydroxide powder is:

Diameter in microns Percent by weight At most 1,11. 49 More than 1,U.at most 3a 23 More than 3,u-at most 5a 19 More than 5,uat most 10 5 More than 10aat most 20 4 More than 20 0 Among inorganic thermal conductive materials, aluminum hydroxide has the greatest effect for the improvement of thermal conductivity, as is evident from FIG. 1. Aluminum hydroxide often contains Fe O and Na O as impurities, Fe O deteriorating the thermal resistance of fiber, Na O impairing electric insulation of the fiber insulator. However, if Fe O and Na O are 0.05% or less and 0.7% or less, respectively, there is no practical harm, as is confirmed by FIGS. 2 to 6. Though most effective in improving the thermal conductivity of insulating paper, non-activated aluminum hydroxide does not absorb impurities in the impregnant or affect the power factor of impregnated insulating paper.

It is particularly important to use non-activated in organic powder in fiber insulators for which impregnation with varnish is contemplated. If activated inorganic powder is employed, the activated powder on or adjacent to the surface of the fiber insulator will absorb solvent and increase the viscosity of the varnish, thus deterring the penetration of said varnish into the interior of the insulator.

Although impregnation from only one side of coarse paper, non-woven fabric and cloth is effective, impregna tion is generally from both sides of the insulator material.

The thermal conductivity of an electrical insulator is also enhanced by impregnating or coating it with a composition (hereinafter: impregnant) which consists essentially of a solution of a varnish and/or a resin, at least one thermal conductive material in the form of a fine powder and a surface active substance.

The solute (varnish and/ or resin) in the solution is any one (or combination) of known resinous or polymeric materials, e.g. shellac, phenolic resin (a condensation product of a phenol, e. g. phenol and cresol, and aldehyde, e.g. formaldehyde and acetaldehyde, which are either of the resol type, i.e. condensed by catalysis of alkali, e.g. NaOH and NH OH, or novolak type condensed by catalysis of acid, e.g. HCl; alcohol is chiefly used as solvent for the former and benzene for the latter) alkyd resin (a condensation product of a polybasic acid, e.g. phthalic acid anhydride, succinic acid, adipic acid, maleic acid and fumaric acid, and a polyhydric alcohol, e.g. ethylene glycol, propylene glyol, glycerine, trimethylol-propane and pentaerythritol, the condensation product being denatured by a drying oil, such as tung oil or linseed oil, by styrene, by rosin or by silicon), polyester resin (polybasic acid/polyhydric alcohol condensate; copolymer of an unsaturated alkyd resin and a compound having a vinyl group; an aryl resin which is a polymer of a polybasic acid/ a monohydric alcohol; or an intermolecular condensate of an oxy-acid), acrylic resin (a polymerization product of acrylic acid and/or its derivative, e.g. an acrylic ester, a methacrylic ester and/or acrylonitrile), polyurethane resin (reaction product of an isocyanate, e.g. tetramethylene diisocyanate and hexamethylene diisocyanate, and a polyether, e.g. polyoxyethyleneglycol or a polyester, e.g. carboxylpolyester), fluorine resin [polymer of fluorine, carbon and chlorine (tetrafluoroethylene resin,

trifluorochloroethylene resin or fiuoroethylene/fluoropropylene copolymer], epoxy resin (a chain polymerizate of epichlorohydrin and a bisphenol or polyhydric alcohol combined with a phenolic resin, combined with a urea resin, esterified with a drying oil fatty acid, esterified with phthalic acid or hardened with a polyamine or polyamide) silicone resin (condensate of an organo chlorosilanol obtained by hydrolysis of an organo chlorosilane and oily resin (obtained by heating and melting together a drying oil, such as tung oil and linseed oil, with a natural resin, e.g. rosin, dammar, fauri, copal and umber, or synthetic resin, e.g. phenolic, alkyd and acrylic) which is soluble in an available solvent.

Suitable solvents for each of the above-noted solutes are known. Illustrative examples of such solvents are:

(l) Phenolic resin:

In case of unmodified resinzalcohol, e.g. ethanol; In case of denatured resinzmixture of solvent naphtha and gasoline; (2) Alkyd resin:

In case of denatured matterzmineral spirts, solvent naphtha, xylol and/or turpentine spirits; In case of being denatured by siliconezxylol; (3) Polyester resin:

Mineral spirits, solvent naphtha, xylol; (4) Acrylic resin:

Ketone, e.g. methylethylethylketone (MEK); (5) Polyurethane resin:

Solvent naphtha, xylol; (6) Fluorine resin:

Alcohol, e.g. ethanol; (7) Epoxy resin:

Ketone, e.g. MEK; hydrocarbon, e.g. benzene and toluene; (8) Silicone resin:

Xylol; (9) Oily resin:

Solvent naphtha, turpentine, benzine.

Inorganic substances of good electrical resistance are those inorganic materials which are only very slightly ionized or unionized in water and which have a specific gravity less than 5, preferably less than 3. The volume resistivity of a dried mixture of said inorganic substances and varnish should be higher than l S2-cm., and preferably higher than 10 Q-cm.

The thermal conductive material comprises from about 20 to about 150 percent by Weight, based on the weight of the solute solids, and preferably has an average particle diameter of at most 0.1 micron (IL). The more finely divided the thermal conductive material, the better the result. The thermal conductive materials are aluminum, titanium, silicon and magnesium oxides and hydroxides, e.g. aluminum hydroxide, titanium dioxide, silica and magnesia; calcium carbonate; clay; talc; basic magnesium carbonate; and aluminum silicate. These my be employed either singly or in any combination and in any proportions.

The surface active substance supplements the thermal conductive material to reduce the surface thermal resistance between the varnish and/or resin and said thermal conductive material. The amount of surface active substance employed is at most percent by weight, based on the weight of the thermal conductive material. Suitable surface active substances are those generally used for other applications; preferred embodiments depend on the varnish and inoranic powder. They include aliphatic amine salts, e.g.

R1\ R\\ RNlI-, llN, m-N

aliphatic ammonium salts, e.g.

polyethyleneglycol esters, e.g.

polyethyleneglycol ethers, e.g. R(O-CH -CH ),,OH; alkylphenol/polyethyleneglycol condensates, e.g.

(I) }CH CHZO)XH polyethyleneglycol alkylamides, e.g. R-C-N CH;CH -O) 1H polyethyleneglycol alkylarnines, e.g. R-N

CH2CE[z-O) H CHCIhOOCR CHOH polyhydrice alcohol esters, e.g. H O

HOH

momo

and ester/ether condensates, e.g.

wherein:

R is an aliphatic hydrocarbon radical, such as alkyl having from 8 to 20 carbons, e.g. octyl, dodecyl, hexadecyl and eicosyl;

each of:

Although no difference is dispersion of intimately and thoroughly admixed thermal conductive material in resin and/0r varnish solutions is observable microscopically whether or not the surface active substance is also incorporated in the dispersion, the surface active substance does reduce the surface thermal resistance between the varnish and/0r resin and the powdered inorganic material. When heat travels through the varnish and/or resin, the said heat passes through part of said varnish and/or resin, through part of the powdered inorganic material and also through surfaces between the varnish and/or resin and the powdered inorganic material. The said surfaces are so many in number that the surface thermal resistance constitutes a large percentage of the total thermal resistance. Table 1 shows the thermal conductivity of (a) alkyd type varnish only, (b) alkyd type varnish mixed with 70 percent by weight, based on varnish solids, of the TABLE 1 Thermal conductivity Article caL/cm. sec. C. (a) 2.8 X 10* (b) 8.4x 10- (c) 10.7 10- wherein R and n have their above-ascribed meanings.

(For this test R is CH (CH CH=CH(CH and An induction motor, 3.7 kw., 200 -v., 3-phase, is obtained by slot insulation (insulation between the coil and the iron core) of the stator, making use of insulating kraft paper having a thickness of 0.25 mm. and containing 30 percent by Weight of non-activated aluminum hydroxide powder (purity: at least 99%; impurities: Na O-0.7% or less, Fe O' 0.05% or less; average diameter: 2.0 microns), and also by impregnation of the stator of the motor with phenolic varnish. A comparison under 100% load between the said motor and an induction motor of the identical type, but subjected to slot insulation by known kraft paper (0.25 mm. thick) and impregnation with phenolic varnish, results in a 44 C. rise in temperature for the motor with known kraft paper insulation and only a 36 C. rise in temperature for the motor with the aluminum hydroxide filled kraft paper insulator.

A comparison of characteristics of insulators prepared from different fibers is provided by the following table:

Composition of pulp Quanti ty (parts by Kind of tFibei and itacgmpositlilon, wgightgigf percen age perccn y wcig t 1(0 3 added to Thermal Dielectric AC break- Cyano- 100 parts conductivity power down Manila ethylated by Weight (cal./en1. Dielectric factor 2 3 voltage 2 Kraft pulp hemp fiber of fiber sec. C.) 1 constant 2 (percent) (kv./mm.)

100 0 2. 82Xl0- 3. 5'; 0.271 8'.1 8. 3. 5 0. 371 80. 2 50 50 0 2. 94 3. 54 0. e12 so. 2 20 8. 0 3. 4 0. 410 75. 8 4 100 0 2. 85 4. 01 0. 42) 77. 5 20 8. l7 3. 86 0. 453 7. 8 6 60 0 2. 91 4. 00 0. 4'33 79. 4 20 8. 29 3. 88 0. 467 74. 7 100 0 2. 95 3. 5'4 0. 27 78. 0 20 8.32 3. 35 0. 412 74. 3

Varnished [alkyd varnish comprising 5% ester surfactant and xylol].

by weight, based on varnish solids, of polybasic acid/polyglycol 4 Nitrogen content .0% by Weight. 5 Nitrogen content 5.1% by Weight.

n is from 2 to 6, inclusive, but replacing said surface active substance with any of those heretofore supported results in a similar effect.)

Microscopic examination reveals no difference between (b) and (c), but it is evident that the surface active substance reduces the surface thermal resistance. The thermal conductivity is thus improved by a combined effect of powdered inorganic material and surface active substance.

Varnished fiber insulator of improved thermal conductivity is thus obtained by coating or impregnating either ordinary insulating paper or the aforementioned insulating paper containing aluminum hydroxide with the impregnant and then drying the same, or by coating or impregnating the aforementioned insulating paper containing aluminum hydroxide of the present invention with a varnish and/or resin solution containing, in intimate admixture therewith, the foregoing surface active substance and then drying the same.

When ordinary insulating paper has dense fibers, powdered inorganic material of the impregnant sometimes fails to penetrate into the core of the paper, thus precluding sufficient improvement in thermal conductivity. However, this problem is not encountered with the aforementioned paper which contains non-activated aluminum hydroxide.

In order to insulate the coil of electrical machinery (insulation between the coil and the earth as well as between layers of the coil), fiber insulator (insulator prepared by admixing non-activated aluminum hydroxide With the pulp slurry in the beater during manufacture) in the form of insulating paper and insulating pressboard and varnished fiber insulator (varnish coated or impregnated fibrous insulator containing thermal conductive inorganic powder and surface active substance) in the form of varnished insulating paper are used.

To prepare impregnant according to this invention, the varnish and/or resin, e.g. at least one of rosin, damrnar, kauri, copal, shellac and amber (natural resins) and phenolformaldehyde (phenolic) resin, alkyd resin, acrylic resin, polyester resin, polyurethane resin, epoxy resin, silicone resin and fluorine-type resin (synthetic resins), is admixed with a solvent, e.g. turpentine spirits, benzine and other petroleum thinners, benzene and the coal tar solvents and higher alcohols, until an appropriate viscosity, e.g. from to 1,600 centipoises or, preferably, from 300 to 1,200 centipoises (the viscosity is suitable for admixture of the surface active substance and the inorganic thermal conductive powder and is thus dependent upon the quantity of inorganic powder) is obtained. Although it is preferred to add the surface active substance first, this is not necessary; the surface active substance and the inorganic powder can be incorporated in the thinned varnish and/ or resin in any order or concurrently. The inorganic powder employed should be dry. The resulting admixture is thoroughly stirred to obtain a uniform non-aqueous composition (impregnant) employed for coating and/or impregnation. The viscosity of said composition can be further adjusted prior to use by the addition thereto of more solvent.

Varnished cloth or paper is obtained by passing the corresponding substrate, i.e. cloth or paper, through a bath of the impregnant two or three times for impregnation or coating and then drying the thus-treated substrate by heating, e.g., at a temperature of from 100 C. to 160 C. (For silicone resin and resin denatured by silicone the preferred drying temperautre is from to 200 C.) This is in accord with established procedures except for the composition of the impregnant.

The cloth is, e.g., Woven or non-woven cloth of cotton, silk, acetate, polyethylene terephthalate fiber or glass fiber.

The insulating impregnant is employed in the same manner and for the same purpose as known materials so used. Both the insulating impregnant and the fiber insulator of this invention are applicable to electric machinery, such as motors, generators, transformers and magnetic coils. The impregnant is useful in the manufacture of electrical appliances, to improve the thermal dispersion of coils of such appliances and to increase the capacity of such electrical appliances.

Resum Thermal conductivity is an important characteristic of electrical insulation as increased thermal conductivity assists in preventing unwanted heat accumulation in, e.g., electrical coils. The thermal conductivity is improved by incorporating finely divided crystalline inorganic thermal conductor in fibrous electrical insulators. This is accomplished through several embodiments:

(I) A varnish and/or resin solution containing, in intimate admixture therewith and dispersed therein, finely powdered crystalline inorganic thermal conductor and surface active substance is impregnated in or coated on fibrous electrical insulation and dried;

(II) Non-activated and inactive finely divided crystalline aluminum hydroxide is intimately admixed with the pulp slurry in the heater in the manufacture of fibrous electrical insulation;

(III) The fibrous electrical insulation of (II) is coated with and/ or impregnated by the varnish and/ or resin solution of (I) and dried; and

(IV) The fibrous electrical insulation of (II) is coated with and/or impregnated by a varnish and/or resin solution containing, in intimate admixture therewith, surface active substance and dried.

EXAMPLES The following examples are presented merely by way of illustration. They demonstrate embodiments of the invention.

EXAMPLE 1 Unbleached kraft pulp is made into an aqueous pulp slurry of approximately 4.5% by weight solids in a hollander-beater, and beaten up to a beating degree of approximately 45 SR (TAPPI-S, T-227 M50). Then nonactivated aluminum hydroxide, e.g. 20 parts by weight per 100 parts by weight of oven dry pulp, is added. The aluminum hydroxide is completely admixed with the pulp by circulation without beating. The resulting mixture is made into insulating paper by known paper manufacturing processes with known paper machinery.

The non-activated aluminum hydroxide is in the form of a powder having an average particle diameter of 1 micron, a purity in excess of 99 percent by weight, an Na O content of at most 0.7 percent by weight and an Fe O content of at most 0.05 percent by weight. The weight percent of aluminum hydroxide, based on kraft pulp solids, is varied from to 50.

Replacing the unbleached kraft pulp with an equivalent of sulfite pulp, manila hemp, linen, ramie, hydroxyethylcellulose or any combination of the foregoing fibers in pulp form results in corresponding products as the percentage of non-activated aluminum hydroxide powder is increased.

Since aluminum hydroxide has a thermal conductivity of about 100 times that of kraft pulp, the thermal conductivity of insulating paper filled with aluminum hydroxide improves according to the amount added, as shown by curve 1 of FIG. 1.

The AC break-down voltage (when insulating oil is impregnated into insulating paper filled with the aluminum hydroxide) is shown by curve 5 of FIG. 2.

The impulse break-down voltage (when insulating oil is impregnated into insulating paper filled with the aluminum hydroxide) is shown by curve 9 of FIG. 3.

The acid value, dielectric power factor and volume resistivity of insulating oil (when 12 g. of ordinary kraft insulating paper or 12 g. of insulating paper filled with 15 percent by weight of non-activated aluminum hydroxide are heated in 250 cc. of insulating oil at C. for 2 days and for 5 days) are shown by (a), (b) and (c) of FIG. 4, respectively. Curve 13 is for insulating paper filled with non-activated aluminum hydroxide, while curve 17 is for an ordinary kraft insulating paper.

FIG. 5 shows the change of water extract conductivity when ordinary kraft insulating paper and insulating paper filled with 15 percent by weight of non-activated aluminum hydroxide are heated in air to 150 C. Curve 18 is for the insulating paper filled with aluminum hydroxide, while curve 22 is for ordinary kraft insulating paper.

FIG. 6 shows the change in pH of water extract of ordinary kraft insulating paper and of insulating paper filled with 15 percent by weight of non-activated aluminum hydroxide when heated in air to 150 C. Curve 23 is for insulating paper filled with aluminum hydroxide, while curve 27 is for ordinary kraft insulating paper.

EXAMPLE 1a Replacing the non-activated aluminum hydroxide of Example 1 with corresponding amounts of non-activated aluminum oxide of the same particle size and purity results in insulating paper of improved thermal conductivity having the characteristics reflected in FIGS. 1 to 6.

The rate of increase in thermal conductivity of the insulating paper with increased aluminum oxide is shown by curve 2 of FIG. 1, while the AC break-down voltage is denoted by curve 6 of FIG. 2 and the impulse breakdown voltage is shown by curve 10 of FIG. 3. The acid value, dielectric power factor and volume resistivity of insulating oil (when 12 g. of insulating paper filled with 15 percent by weight of non-activated aluminum oxide is heated in 250 cc. of insulating oil at 150 C. for 2 days and for 5 days) are shown by 14 of a, b and c of FIG. 4, respectively. The change in water extract conductivity and the change in pH of water extract (when the insulating paper filled with 15 percent by weight of non-activated aluminum oxide is heated in air to 150 C.) are shown by curve 19 of FIG. 5 and curve 24 of FIG. 6, respectively.

EXAMPLE 1b Replacing the non-activated aluminum hydroxide of Example 1 with corresponding amounts of non-activated calcium carbonate powder comprised of particles varying from 0.5-1 micron in diameter results in insulating paper of improved thermal conductivity having the characteristics reflected in FIGS. 1 to 6.

The rate of increase in thermal conductivity of the insulating paper with increased calcium carbonate is shown by curve 3 of FIG. 1, while the AC break-down voltage is denoted by curve 7 of FIG. 2 and the impulse breakdown voltage is denoted by curve 11 of FIG. 3. The acid value, dielectric power factor and volume resistivity of insulating oil (when 12 g. of insulating paper filled with 15 percent by weight of non-activated calcium carbonate are heated in 250 cc. of insulating oil at 150 C. for 2 days and for 5 days) are shown by 15 of a, b and c of FIG. 4, respectively. The change in water extract conductivity and the change in pH of water extract (when the insulating paper filled with 15 percent by weight of non-activated calcium carbonate is heated in air to 150 C.) are shown by curve 20 of FIG. 5 and curve 25 of FIG. 6, respectively.

EXAMPLE 1c Replacing the non-activated aluminum hydroxide of Example 1 with corresponding amounts of non-activated talc powder comprised of particles varying from 0.5-2 microns in diameter results in insulating paper of improved thermal conductivity having the characteristics reflected in FIGS. 1 to 6.

The rate of increase in thermal conductivity of the insulating paper with increased talc is denoted by curve 4 of FIG. 1, and the AC break-down voltage is indicated by curve 8 of FIG. 2, while the impulse break-down voltage is shown by curve 12 of FIG. 3. The acid value, dielectric power factor and volume resistivity of insulating oil (when 12 g. of insulating paper filled with 15 percent by weight of non-activated talc are heated in 250 cc. of insulating oil at 150 C. for 2 days and for days) are shown by 16 of a, b and c of FIG. 4, respectively. The change in Water extract conductivity and the change in pH of water extract (when the insulating paper filled with percent by weight of non-activated talc is heated in air to 150 C.) are shown by curve 21 of FIG. 5 and curve 26 of FIG. 6, respectively.

FIGS. 1 to 6 illustrate that non-activated aluminum hydroxide powder of the indicated purity and particle size is the best filler for improving the thermal conductivity of electrical insulation without impairing the electric and other essential characteristics of the electrical insulation.

EXAMPLE 2 A non-woven fabric (or synthetic paper) having a thickness of 0.07 millimeters (mm.) and made of polyethylene terephthalate fiber is coated with a stirred mixture of impregnant of the following composition:

Parts by weight, based on 100 Ingredient: parts of varnish solids Non-activated aluminum hydroxide thermal conductive powder 70 Polyglycol/polybasic acid ester surface active substance Urethane-type varnish 100 The particles of the aluminum hydroxide powder have a diameter of about 50 millimicrons. Said aluminum hydroxide otherwise satisfies the requirements of that employed in Example 1. The surface active substance is the same as that employed to obtain the data of Table 1.

The urethane-type varnish is based on urethane resin prepared by condensing tetramethylene diisocyanate with propylene oxide. Said varnish consists of, e.g., commercially available urethane resin dissolved in 2-3 times its Weight of an admixture of xylol and solvent naphtha as solvent.

The thus-coated fabric is dried, yielding a dried varnished fabric having a thickness of 0.13 mm. and good thermal conductivity. The thermal conductivity of the resulting varnished fabric is compared (Table 2) with that of varnished fabric which is otherwise identical except for the omission therefrom of the thermal conductive powder and the surface active substance.

TABLE 2 Thermal conductivity Article: cal./cm.sec. C.

Varnished non-woven fabric of Example 2 4.7 10- Ordinary varnished non-woven fabric 2.6 10- The change in AC insulation break-down voltage when varnished non-woven fabric of good thermal conductivity of this example is heated in air to 130 C. is shown in FIG. 7, wherein the abscissa denotes the heating time and the ordinate represents AC break-down voltage. Curve 28 indicates change in the AC break-down voltage due to heating the varnished non-woven fabric of this example.

FIG. 8 shows the change in volume resistivity of varnished non-woven fabric of good thermal conductivity of this example when the fabric is heated in air to 130 C. The abscissa denotes heating time and the ordinate represents volume resistivity. Curve 29 denotes change in volume resistivity due to heating said varnished non-woven fabric.

FIG. 9 shows AC insulation break-down voltage of said varnished non-woven fabric when it is placed in air having relative humidity at room temperature. The abscissa denotes moisture absorption time and the ordinate represents AC insulation break-down voltage. Curve 30 denotes change in AC breakdown voltage due to moisture absorption of said varnished fabric.

FIG. 10 shows change in volume resistivity when the varnished non-woven fabric of good conductivity of this example is placed in air having 100% relative humidity. The abscissa denotes moisture absorption time and the ordinate represents volume resistivity. Curve 31 shows change in volume resistivity due to moisture absorption of the varnished non-woven fabric of good thermal conductivity of this example.

Replacing the varnish with a corresponding amount (based on solids) of shellac or phenolic varnish results in a similar comparison of thermal conductivity between coated fabrics containing and those lacking the thermal conductive powder and surface active substance. The final products in each case are identical in all respects other than the type of varnish employed.

Replacing the non-activated aluminum hydroxide powder with a like amount of alumina, silica, clay, talc or basic magnesium carbonate (each having an average particle diameter of at most 0.1 micron) also improves the thermal conductivity of varnished fabric otherwise identical to that of this example. The surface active substance/ resin combination is of the essence in this example, i.e. irrespective of the activation or non-activation of the aluminum hyroxide or other filler.

EXAMPLE 3 Kraft insulating paper filled with non-activated aluminum hydroxide powder is prepared in strict accord with Example 1 except that the aluminum hydroxide comprises 40 percent by Weight of the insulating paper. The thusfilled kraft insulating paper is coated with oily varnish containing 2 parts by weight of polyethyleneglycol/polybasic acid ester surface active substance per 100 parts by Weight of varnish solids. The coated kraft paper is then dried to produce a varnished paper of 0.13 mm. thickness having good thermal conductivity.

The surface active substance is the same as that employed in Example 2. The varnish, however, is based upon alkyd resin heated in admixture with melted linseed oil. Said varnish consists of the resin dissolved in an equal weight of turpentine spirits, as solvent.

A comparison between the thermal conductivity of the thus-prepared insulating paper and that of varnished paper identical thereto except for the absence therein of aluminum hydroxide and surface active substance is provided in Table 3.

TABLE 3 Thermal conductivity Article: cal./cm.sec. C. Varnished paper of Example 3 3.1 10- Ordinary varnished paper 1.9 10- Replacing the surface active substance with either a polyethyleneglycol alkylaminc, e.g. that of the formula mal conductivity comparisons with ordinary varnished paper.

15 EXAMPLE 4 Electrical insulating paper made from manila hemp and having a thickness of 0.05 mm. is coated With a stirred mixture of 70 parts by weight (based on 100 parts by weight of varnish solids) of the same non-activated aluminum hydroxide powder used for Example 2, 2 parts by weight of the surface active substance employed in Example 2 and 100 parts by weight of the same oily varnish as employed for Example 3 (all parts being on the same basis) and dried. The resulting dry varnished paper has a thickness of 0.1 mm. and good thermal conductivity.

A comparison between the thermal conductivity of the thus-prepared insulating paper and that of varnished paper identical thereto except for the aluminum hydroxide and the surface active substance in the impregnant is provided by Table 4.

TABLE 4 Thermal conductivity Article: cal./cm.sec. C. Varnished paper of Example 4 3.8 l Ordinary varnished paper 2.1

Replacing the aluminum hydroxide powder with a like amount of either aluminum oxide, titanium dioxide, magnesia, calcium carbonate or aluminum silicate (each having an average particle diameter of at most 0.1 micron) results in a similar comparison in thermal conductivity between coated papers containing and those lacking the inorganic powder and surface active substance.

FIG. 11 shows the change in AC insulation breakdown voltage when varnished paper of good thermal conductivity of Example 3 is heated in air to 110 C. The abscissa denotes heating time, while the ordinate shows AC insulation break-down voltage. Curve 32 illustrates the results when using the product of Example 3, while curve 33 represents the product of Example 4.

FIG. 12 shows the change in volume resistivity when varnished paper of good thermal conductivity is heated in air to 110 C. The abscissa denotes heating time, while the ordinate shows volume resistivity. Curve 34 represents the product of Example 3, while curve 35 represents that of Example 4.

EXAMPLE 5 Coil insulating paper having a thickness of 130 microns is prepared according to the procedure of Example 1 by adding 30 parts by weight of the same aluminum hydroxide powder as employed in Example 2 to 100 parts by weight of kraft pulp (all parts by weight being on a dry basis) in the beater. The resulting coil insulating paper of good thermal conductivity and ordinary coil insulating paper (lacking the aluminum hydroxide) were separately used (with the aid of 0.45 mm. enamel copper wire) to prepare cylindrical coils (having an interior diameter of mm. and 15 layers for each 80 turns) of the same size and shape.

The two coils were placed upon adiabatic materials and connected in series. The DC current of 1 ampere was applied thereto to measure the change in voltage at both ends of the respective coils to find out the change in conductor resistance from the increase of which the average temperature rise of the coil conductors is determined. The results are shown in FIG. 13, wherein the abscissa denotes time, while the ordinate shows average temperature rise. Curve 37 is for the coil made with coil insulating paper of good thermal conductivity and curve 36 is for the coil made with ordinary coil insulating paper. In FIG. 13 the average temperature rise of the coil made with coil insulating paper of good conductivity of the present invention is about 15% lower than that for the coil made with ordinary coil insulating paper. Therefore it is evident that the insulating paper of good thermal conductivity of the present invention is of great practical use.

1% EXAMPLE 6 The surface active substance is of material import in the impregnant compositions and in the impregnated fibrous insulators. When heat travels through varnished insulating materials, it passes through the varnish part, the fiber part, the powdered inorganic substance part and through the surfaces between the varnish and the fiber or the powdered inorganic substance. Since each of the foregoing parts has a specific thermal conductivity and the thermal conductivity of the powdered inorganic substance is generally greater than that of either the varnish or the fiber, the thermal conductivity of the material which contains powdered inorganic substance is greater than that of material which contains none of the powdered inorganic substance. However, as heat passes through a number of surfaces between the varnish and fiber or powdered inorganic substance, the surface thermal resistance accounts for a considerable percentage of the overall thermal resistance. This surface thermal resistance is reduced by surface active substance. The thermal resistance of the insulator as a whole is, therefore, further diminished by surface active substance. When two samples are prepared under identical conditions except that one contains surface active substance therein and the other contains no surface active substance, a microscopic examination revals no difference between the two samples as far the construc tion and the condition of dispersion of the powdered inorganic substance. However, the thermal conductivity of the sample which contains surface active substance is approximately 20% greater than that which contains no surface active substance.

(a) Add parts by weight, based on parts by weight of varinsh resin solids, of powdered non-activated aluminum hydroxide having an average particle diameter of 50 millimicrons and 3 parts by weight, based on 100 parts by weight of the aluminum hydroxide, of polyoxyethylene alkylamine (surface active substance) to polyurethane resin insulating varnish. The resin solids are about 55 percent by weight, and the solvent is xylol. The sulting admixture is thoroughly stirred to obtain a uniform impregnant.

(b) Prepare a composition identical to that of (a) except for the omission therefrom of the surface active substance.

(c) Polyethylene terephthalate cloth having a thickness of 0.07 mm. is cut into swatches approximately 40 mm? and perfectly vacuum impregnated with one of the two preparations (a) and (b). Ten swatches impregnated with (a) are piled one upon the other, sandwiched between two metal plates (each 40 mrn. by 0.5 mm. thick) and dried by heating under a fixed load. Ten swatches impregnated with (b) are treated in the identical manner. The thermal conductivity of (a) is 6.94 l0 cal/cm. sec. C., Whereas the thermal conductivity of (b) is only 5.63 10- cal/cm. sec. C. This significant difference is attributed to the presence of the surface active substance. Said surface active substance actually reduces surface thermal resistance without adversely affecting the electrical insulation characteristics of varnish. Essentially the same results are obtained when the non-activated aluminum hydroxide powder is replaced by (otherwise identical) activated aluminum hydroxide powder.

The invention and its advantages are readily understood from the preceding description. Various changes may be made in the process and products without departing from the spirit and scope of the invention or sacrificing its material advantages, the products and processes hereinbefore described being merely illustrative embodiments.

What is claimed is:

1. A method of improving the thermal conductivity of a fibrous electrical insulator selected from the group consisting of insulating paper, pressboard and non-woven fabric, for use in electrical apparatus other than condensers, which comprises intimately and coextensively incorportaing in the insulator from 10 to 50% by weight,

1 7 based on the weight of the improved insulator, of nonactivated aluminum hydroxide powder having an average particle diameter of at most several microns, a purity of at least 99% by weight, an Na O content of at most 0.7% by weight and an Fe O content of at most 0.05% by weight.

2. A method of improving the thermal conductivity of varnish for electrical insulation for use in electrical apparatus other than condensers, which comprises incorporating in the varnish (1) from 20 to 150% by weight of the varnish solids of a nonactivated, inorganic, crystalline, thermal conductive powder having an Na O content of at most 0.7% by weight and an Fe O content of at most 0.05% by weight and (2) at most 5% by weight of the varnish solids of a surface active substance from the group consisting of aliphatic amine salts, aliphatic ammonium salts, polyethyleneglycol esters, polyethyleneglycol ethers, allsylphenol/polyethyleneglycol condensates, polyethyleneglycol alkylamides, polyethylene glycol alkylamines, polyhydric alcohol esters and ester/ether condensates.

'3. The method according to claim 2, wherein the thermal conductive powder is selected from the group 2O HAROLD ANSHER,

consisting of aluminum hydroxide, aluminum oxide, calcium carbonate, magnesium oxide, basic magnesium carbonate, silica, clay, talc, titanium oxide and aluminum silicate.

References Cited UNITED STATES PATENTS 2,281,602 5/1942 Ruben 317-258 2,460,126 1/1949 Clark 252-635 2,550,452 4/1951 Byrne et al. 317-258 2,594,872 4/1952 Clark 317-259 2,594,873 4/1952 Clark 317-259 2,921,867 1/1960 Shaw 117-103 2,975,147 3/1961 Abbott et al. 260-75 3,026,241 3/1962 Hechtman et al. 162-135 3,083,119 3/1963 Flowers et al. 117-155 3,090,705 5/1963 Miksits 117-154 3,416,942 12/1968 Scutzner 117-154 X Primary Examiner US. Cl. X.R. 

